The vast majority of diseases that cause catastrophic loss of vision do so as a result of ocular neovascularization. For example, age related macular degeneration (ARMD) affects 12-15 million American over the age of 65 and causes visual loss in 10-15% of them as a direct effect of choroidal (sub-retinal) neovascularization. The leading cause of visual loss for Americans under the age of 65 is diabetes; 16 million individuals in the United States are diabetic and 40,000 per year suffer from ocular complications of the disease, often a result of retinal neovascularization. While laser photocoagulation has been effective in preventing severe visual loss in subgroups of high risk diabetic patients, the overall 10-year incidence of retinopathy remains substantially unchanged. For patients with choroidal neovascularization due to ARMD or inflammatory eye disease such as ocular histoplasmosis, photocoagulation, with few exceptions, is ineffective in preventing visual loss. While recently developed, non-destructive photodynamic therapies hold promise for temporarily reducing individual loss in patients with previously untreatable choroidal neovascularization, only 61.4% of patients treated every 3-4 months had improved or stabilized vision compared to 45.9% of the placebo-treated group.
ARMD and diabetic retinopathy are the leading causes of visual loss in industrialized nations and do so as a result of abnormal retinal neovascularization. Since the retina consists of well-defined layers of neuronal, glial, and vascular elements, relatively small disturbances such as those seen in vascular proliferation or edema can lead to significant loss of visual function. Inherited retinal degenerations, such as retinitis pigmentosa (RP), are also associated with vascular abnormalities, such as arteriolar narrowing and vascular atrophy. While significant progress has been made in identifying factors that promote and inhibit angiogenesis, no treatment is currently available to specifically treat ocular vascular disease.
Inherited degenerations of the retina affect as many as 1 in 3500 individuals and are characterized by progressive night blindness, visual field loss, optic nerve atrophy, arteriolar attenuation, altered vascular permeability and central loss of vision often progressing to complete blindness (Heckenlively, J. R., editor, 1988; Retinitis Pigmentosa, Philadelphia: JB Lippincott Co.). Molecular genetic analysis of these diseases has identified mutations in over 110 different genes accounting for only a relatively small percentage of the known affected individuals (Humphries et al., 1992, Science 256:804-808; Farrar et al. 2002, EMBO J. 21:857-864.). Many of these mutations are associated with enzymatic and structural components of the phototransduction machinery including rhodopsin, cGMP phosphodiesterase, rds peripherin, and RPE65. Despite these observations, there are still no effective treatments to slow or reverse the progression of these retinal degenerative diseases. Recent advances in gene therapy have led to successful reversal of the rds (Ali et al. 2000, Nat. Genet. 25:306-310) and rd (Takahashi et al. 1999, J. Virol. 73:7812-7816) phenotypes in mice and the RPE65 phenotype in dogs (Acland et al. 2001, Nat. Genet. 28:92-95) when the wild type transgene is delivered to photoreceptors or the retinal pigmented epithelium (RPE) in animals with a specific mutation.
Angiogenesis is the process by which new blood vessels form. In response to specific chemical signals, capillaries sprout from existing vessels, eventually growing in size as needed by the organism. Initially, endothelial cells, which line the blood vessels, divide in a direction orthogonal to the existing vessel, forming a solid sprout. Adjacent endothelial cells then form large vacuoles and the cells rearrange so that the vacuoles orient themselves end to end and eventually merge to form the lumen of a new capillary (tube formation).
Angiogenesis is stimulated by a number of conditions, such as in response to a wound, and accompanies virtually all tissue growth in vertebrate organisms such as mammals. Angiogenesis also plays a role in certain disease states such as certain cancers. The growth of tumors, for example, requires blood vessel growth to provide oxygen and nutrients to the growing tumor tissue. In addition, ocular neovascularization is associated with the vast majority of eye diseases that lead to catastrophic loss of vision.
Angiogenesis may be arrested or inhibited by interfering with the chemical signals that stimulate the angiogenic process. For example, angiogenic endothelial cells produce proteases to digest the basal lamina that surround the blood vessels, thus clearing a path for the new capillary. Inhibition of these proteases, or their formation, can prevent new vessels from forming. Likewise, the endothelial cells proliferate in response to chemical signals. Particularly important proliferation signals include the vascular endothelial growth factor (VEGF), and the fibroblast growth factor (FGF) families of proteins. VEGF has been shown to be involved in vascularization of certain tumors. Interference with these proliferation signaling processes can also inhibit angiogenesis.
Several factors are involved in angiogenesis. Both acidic and basic fibroblast growth factor molecules are mitogens for endothelial cells and other cell types. A highly selective mitogen for vascular endothelial cells is VEGF.
In the normal adult, angiogenesis is tightly regulated, and is limited to wound healing, pregnancy and uterine cycling. Angiogenesis is turned on by specific angiogenic molecules such as basic and acidic fibroblast growth factor (FGF), VEGF, angiogenin, transforming growth factor (TGF), tumor necrosis factor-α (TNF-α) and platelet derived growth factor (PDGF). Angiogenesis can be suppressed by inhibitory molecules such as interferon-α, thrombospondin-1, angiostatin and endostatin. It is the balance of these naturally occurring stimulators and inhibitors that controls the normally quiescent capillary vasculature. When this balance is upset, as in certain disease states, capillary endothelial cells are induced to proliferate, migrate and ultimately differentiate.
Angiogenesis plays a central role in a variety of disease including cancer and ocular neovascularization. Sustained growth and metastasis of a variety of tumors has also been shown to be dependent on the growth of new host blood vessels into the tumor in response to tumor derived angiogenic factors. Proliferation of new blood vessels in response to a variety of stimuli occurs as the dominant finding in the majority of eye disease and that blind including proliferative diabetic retinopathy, ARMD, rubeotic glaucoma, interstitial keratitis and retinopathy of prematurity. In these diseases, tissue damage can stimulate release of angiogenic factors resulting in capillary proliferation. VEGF plays a dominant role in iris neovascularization and neovascular retinopathies. While reports clearly show a correlation between intraocular VEGF levels and ischemic retinopathic ocular neovascularization, FGF likely plays a role as well. Basic and acidic FGF are known to be present in the normal adult retina, even though detectable levels are not consistently correlated with neovascularization. This may be largely due to the fact that FGF binds very tightly to charged components of the extracellular matrix and may not be readily available in a freely diffusible form that would be detected by standard assays of intraocular fluids.
A final common pathway in the angiogenic response involves integrin-mediated information exchange between a proliferating vascular endothelial cell and the extracellular matrix. This class of adhesion receptors, called integrins, are expressed as heterodimers having an α and β subunit on all cells. One such integrin, αvβ3, is the most promiscuous member of this family and allows endothelial cells to interact with a wide variety of extracellular matrix components. Peptide and antibody antagonists of this integrin inhibit angiogenesis by selectively inducing apoptosis of the proliferating vascular endothelial cells. Two cytokine-dependent pathways of angiogenesis exist and may be defined by their dependency on distinct vascular cell integrins, αvβ3 and αvβ5. Specifically, basic FGF- and VEGF-induced angiogenesis depend on integrin αvβ3 and αvβ5, respectively, since antibody antagonists of each integrin selectively block one of these angiogenic pathways in the rabbit corneal and chick chorioallantoic membrane (CAM) models. Peptide antagonists that block all αv integrins inhibit FGF- and VEGF-stimulated angiogenesis. While normal human ocular blood vessels do not display either integrin, αvβ3 and αvβ5 integrins are selectively displayed on blood vessels in tissues from patients with active neovascular eye disease. While only αvβ3 was consistently observed in tissue from patients with ARMD, βvβ3 and αvβ5 both were present in tissues from patients with proliferative diabetic retinopathy. Systemically administered peptide antagonists of integrins blocked new blood vessel formation in a mouse model of retinal vasculogenesis.
Hence, anti-angiogenic agents have a role in treating retinal degeneration to prevent the damaging effects of these trophic and growth factors. Angiogenic agents, also have role in promoting desirable vascularization to retard retinal degeneration by enhancing blood flow to cells.
Immense research efforts have contributed to our understanding of the mechanisms of angiogenesis during disease progression, and as a result of these studies, a large number of angiostatic molecules have been, or are currently being, tested in clinical trials. However, to date, the results from these clinical trials have been disappointing, and the benefits from these antiangiogenic treatments in patients have been minimal at best.
Many factors may require consideration before angiostatic therapies ultimately become successful. Naturally occurring compensatory mechanisms may ultimately render angiogenic monotherapies obsolete. Angiostatic drugs generally target a single cytokine or intracellular angiogenic pathway. In vivo, angiogenesis is likely to be initiated by the combined signaling of multiple pathways. Thus, blocking a single pathway may be insufficient to prevent angiogenesis during the treatment of neovascular diseases. Further complicating matters, it is also likely that blocking a single pathway induces compensation and increased roles of other angiogenic pathways.
It has now been discovered that a concurrent administration of a combination of angiostatic compounds that target different pathways enhances angiostatic potency and also interferes with natural compensatory mechanisms.